Prosecutors in Minnesota’s Hennepin County are relying on a little-used law to charge drug dealers with third-degree murder when sales result in overdose deaths.

Hennepin County prosecutors have charged six defendants with third-degree murder for drug deaths in the last 18 months, though not all of them were accused drug dealers, the Minneapolis Star Tribune reports. In 2008 through 2010, on the other hand, prosecutors in the county didn’t bring any cases under the law. Washington County prosecutors have filed third-degree murder charges twice this year and as many as four times last year.

One hurdle: Prosecutors must be able to link the defendant to the overdose victim. In one recent case, prosecutors used surveillance footage of a drug buy and DNA evidence on a baggie in the victim’s pocket to charge Devon McFerrin, 31, with third-degree murder, the story says. A warrant has been issued for his arrest.

The Hennepin County prosecutions have resulted in just one conviction. The defendant, Melanie Razo of Eden Prairie, was accused of talking her husband into trying heroin and failing to quickly call 911 when he collapsed, the story says. She was sentenced to one year in the county workhouse.

The new prosecutions are a response to “an alarming rise in heroin overdose deaths,” the newspaper says. In 2011, a record 120 people died of heroin or opiate overdoses in Minneapolis and St. Paul.

Under federal law, heroin dealers face an increased punishment of up to life in prison when the drug user dies. The U.S. Supreme Court on Monday agreed to decide whether the increased penalty applies when the heroin contributed to the death, or if it must be the proximate cause of death, SCOTUSblog reports. The case is Burrage v. United States.